The application of lasers and fiber optics to communications and high speed data transfer has created a need for high speed optical modulators to take advantage of the wide bandwidth available at optical frequencies. Optical modulators such as Mach-Zender interferometers are needed to implement coherent PSK or FSK modulation in long haul optical fiber transmission systems. Directional couplers and switches are also needed for optical switching, multiplexing, and high speed signal processing, such as in analog to digital conversion. There are also a growing number of non-communication applications for high speed optical modulators, such as high speed optical based image or data processing, or logic.
Several types of modulators have been developed using electro-optic crystals, electro-absorptive materials, and materials with an electrically alterable index of refraction. Recent development efforts have generally been directed to providing reduced modulator size, increased modulation efficiency, and increased speed.
A common type of optical modulator is a phase modulator made from LiNbO.sub.3 electro-optic material. At high frequencies, the r33 component of an electro-optic tensor in a LiNbO.sub.3 crystal structure is very large, typically on the order of 3.08.times.10.sup.-9 cm/Volt. Using this effect and known relationships between applied voltages along the Z crystal axis, crystal dimensions, and modulator configuration, the index of refraction for an extraordinary ray polarized along the Z direction can be altered by changing the magnitude of an electric field applied across the crystal.
In order to obtain complete intensity modulation and a very efficient design, the light must have its phase shifted by a factor of .pi.. The product of the length of the modulation path and the applied drive voltage required for a .pi. phase shift represents a figure of merit for classifying such devices. A second figure of merit for such devices is the ratio of required drive voltage and device bandwidth. For LiNbO.sub.3, this value is typically optimized to 1.5-2.4 Volts per Gigahertz at 1.3 .mu.m wavelength and 2.5 GHz. However, implementation of electro-optical phase modulators requires very high geometric precision. At the same time, modulator configurations must be cautiously designed and adjusted to avoid stray capacitances and parasitic resistances which degrade speed and switching performance. The requirements for precision and optimization increase manufacturing complexity and cost while decreasing reproducibility.
As an alternative, it has been shown that higher bandwidths can be achieved using traveling-wave type modulators where the necessary drive voltage decreases with increasing length. One of the best reported traveling-wave modulators has a bandwidth of 11.2 GHz, a length of 0.6 cm, and a drive voltage of 8.8 volts. The bandwidth product of 6.7 GHz-cm is close to a 9 GHz-cm theoretical maximum for the material. While this type of modulator can achieve high modulation rates, inherent propagation delays due to the longer length make it unsuitable for optical logic.
Direct modulation of optical intensity is also accomplished using the electro-absorption effect in semiconductors. In this technique, the energy level of the band edge in an optical transmission material is decreased by an increase in an applied electric field. This makes it possible to affect modulation when optical radiation being transferred through the modulator is tuned in frequency near the band edge. The electro-absorptive effect has also been implemented using multiple quantum well material and Self-Electro-optic-Effect Devices (SEED). However, because these devices run near the band edge, they are very sensitive to both wavelength and temperature variations. Small shifts in either parameter have a large impact on the transfer or modulation of radiation which quickly degrades performance. Switching times for electro-absorption devices also tend to be very slow for the applications of interest.
Another technique is to use the effects of free carriers on waveguide transmission properties. A group of charged carriers are introduced from outside of a waveguide region to alter the index of refraction of the material forming the waveguide region. This can be done by applying an electric field to a material adjacent to the waveguide to move charged carriers, such as electrons, into the waveguide region. However, introduction of charge carriers is generally a slow process requiring carrier transfer over substantial distances on the device scale. There are inherent problems with switching speed resulting from maximum carrier velocity or rate of diffusion within a material structure and subsequent recombination outside of the waveguide region. Therefore, no matter how fast the carrier transfer is driven, inherent time delays set a limit on the resolution or response time of the modulator. At the same time, the threshold for carrier or electron motion also sets a loss in terms of energy extracted from an optical signal.
What is needed then is a new method of operation or apparatus for modulating optical signals which reduces signal loss and operates at high speeds. The apparatus needs to be implemented with minimal complexity and precision constraints. It is also desirable to minimize driving voltage and device capacitance while matching low impedance input sources. Any new device should be highly compatible with advanced laser and Multiple Quantum Well (MQW) devices.